Senator Bernie Sanders shocked the political establishment by winning 13 million votes and a majority of young voters in the 2016 Democratic primary. He emerged from the contest against Hillary Clinton as the most popular politician in the US, despite being a 75-year-old self-professed "democratic socialist." What lessons can be drawn from this surprising but-in the end-losing campaign? Vermont native Heather Gautney was a legislative fellow in Sanders's Washington office and a senior researcher on his presidential campaign. The author and editor of several books on social movements and American politics, she brings her academic expertise and left politics to bear on the scenes and conflicts she witnessed from inside the campaign. In reviewing what enabled Sanders to reach out to an unprecedented number of people with a socialist message-and what stalled his progress and radical punch-she draws lessons about the prospects and perils of building a leftist movement in the United States. Gautney's reflections on the role that race and class played in this election cycle and analysis of where Democrats stand following Trump's victory will serve as a useful starting point for many newly aware of the limitations of the Democratic party and the challenges ahead.